ADU Law Update: What It Means for Cornish Home Values

Thinking about adding a small second unit to your Cornish home or buying a place with extra space for family or rental income? New Hampshire’s updated ADU law, effective July 1, 2025, makes it easier than ever to build or legalize accessory dwelling units. If you are wondering what that means for your home’s value, you are not alone. In this guide, you will learn what changed, how it applies in Cornish, and practical steps to protect and grow your equity. Let’s dive in.

What changed in NH’s ADU law

New Hampshire passed HB 577, a statewide update that expands what towns must allow for accessory dwelling units. The law requires municipalities to permit one ADU by right in any zone where single‑family homes are allowed. Here are the key points:

• Detached ADUs are allowed, not just attached ones.
• Towns cannot require more lot size, frontage, or setbacks than they require for a single‑family home, with limited exceptions.
• The default ADU size cap is 950 square feet, and towns cannot set a maximum below 750 square feet.
• Towns cannot require a separate water or septic system beyond state rules, though you still must meet state standards and obtain approvals when needed.
• Towns may require owner‑occupancy of one unit and up to one extra parking space if parking is already required for the main home. Aesthetic standards must match those for the principal home.

What this means in Cornish

Cornish is a small, mostly owner‑occupied community with an older median age and a rural feel. (Town snapshot) If Cornish’s zoning previously limited or banned ADUs, those limits are now preempted. One ADU is permitted by right, and local procedures will be updated to align with the state statute. The town can still apply neutral development standards that apply to both the main home and the ADU combination, and it may set owner‑occupancy or parking rules within the state law’s limits. For local implementation steps, New Hampshire Municipal Association offers useful guidance for officials and boards on aligning ordinances with the law. (Municipal guidance)

Most Cornish homes rely on private wells and on‑site septic. The state law does not waive health and safety rules. If an ADU changes the number of bedrooms or increases design flow, you may need an NHDES approval or a septic upgrade. NHDES standards control design flows, system sizing, and approvals. (NHDES septic rules)

Likely impact on home values

Research shows that permitted, well‑designed ADUs tend to make homes more marketable and can raise appraised value. The size of the uplift varies by market, construction cost, and rental demand. National studies often cite a range of roughly 10 to 35 percent, with the higher end seen in strong urban markets. (ADU research overview)

Cornish is likely to see a more modest premium than large cities. Factors that can limit the uplift include a smaller rental market, potential septic upgrade costs, owner‑occupancy rules, and fewer local comps with ADUs to guide appraisals. Even so, a permitted ADU can expand your buyer pool and support a stronger price position, especially if the unit is turnkey and well documented.

Who benefits most locally

• Homeowners who want space for multigenerational living or caregiving with privacy.
• Owners who want to offset costs with long‑term rental income, where allowed.
• Buyers who value flexibility for a studio, office, or guest suite with separate entry.
• Sellers with documented, permitted ADUs that appraisers can credit with confidence.

Step‑by‑step if you are considering an ADU

1. Confirm local procedures. Check with Cornish’s building or planning office on current forms, inspections, and any owner‑occupancy or parking rules. The statewide framework is set by HB 577, but local processes still apply.
2. Evaluate septic and water. If the ADU will add bedrooms or increase design flow, a licensed septic designer should assess capacity, approvals, and potential upgrade costs under NHDES rules.
3. Plan for permits and documentation. A permitted ADU is far more likely to be fully credited in an appraisal and to reassure buyers.
4. Vet the value. Ask a local appraiser or agent for a paired‑sales analysis and, if relevant, rental evidence. National research supports value gains, but local comps and costs determine your property’s outcome. (Market context)

Selling a Cornish home with an ADU

To maximize price, present the ADU clearly and professionally. Gather permits, final inspections, floor plans, bedroom counts, and any rental history. Highlight utility set‑ups, parking, and access. Premium marketing, staging guidance, and narrative‑driven presentation help buyers picture how they will use the space, which supports stronger offers and smoother appraisal.

LIVE UV LIFE offers concierge‑level listing services through Berkshire Hathaway HomeServices Verani Realty, including professional photography, staging advice, and targeted digital marketing, paired with negotiation expertise. That combination helps you showcase the ADU’s value while managing details that matter to appraisers and lenders.

Buying a Cornish home with ADU potential

If you are shopping for properties that could support an ADU, check setbacks, lot coverage, and access alongside septic capacity and well placement. Confirm any owner‑occupancy rules and parking standards. Ask for prior approvals or septic plans if a bedroom count change is likely. The new law provides a clearer path, and a good team can help you separate easy wins from costly surprises.

Bottom line

A permitted ADU in Cornish will likely make your home more appealing and can support a price premium, though the uplift is usually smaller than in high‑rent cities. The best results come from clear permits, smart design, and a tailored marketing plan that speaks to how buyers want to live here in the Upper Valley.

Ready to plan your ADU strategy or pricing? Connect with Lori Shipulski for a local, data‑driven plan that protects your investment and optimizes your next move.

FAQs

Are ADUs now allowed in Cornish, NH?

• Yes. As of July 1, 2025, state law requires towns to allow one ADU by right in single‑family zones, with limited local rules on owner‑occupancy, parking, and aesthetics. (HB 577)

Do I need a new septic system for an ADU in Cornish?

• Not always. Towns cannot require a separate system beyond state standards, but if bedrooms or design flow increase, NHDES approvals or upgrades may be needed. (NHDES rules)

How will an ADU affect my Cornish home’s value?

• Nationally, ADUs often add value, commonly cited around 10–35 percent; Cornish is likely toward the lower end due to rural demand and septic costs. Use local comps and an appraiser. (Research summary)

Can I rent my Cornish ADU short‑term or long‑term?

• Long‑term rentals may be allowed, but owner‑occupancy and local short‑term rental rules can apply. Always confirm current Cornish ordinances and policies. (HB 577 overview)

Will adding an ADU raise my property taxes in Cornish?

• Likely yes, because added living area can increase assessed value; no Cornish‑specific tax break was identified, though some areas elsewhere have incentives. (Example coverage)